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<html>
  <head>
    <meta charset='utf-8'>
    <meta name='viewport' content='width=device-width, initial-scale=1, user-scalable=no'>
    <meta name='mobile-web-app-capable' content='yes'>
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    <!-- Origin Trial Token, feature = WebXR Device API, origin = https://immersive-web.github.io, expires = 2018-08-28 -->
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    <title>Reduced Bind Rendering</title>

    <link href='css/common.css' rel='stylesheet'></link>

    <!--The polyfill is not needed for browser that have native API support,
        but is linked by these samples for wider compatibility.-->
    <!--script src='https://cdn.jsdelivr.net/npm/webxr-polyfill@latest/build/webxr-polyfill.js'></script-->
    <script src='js/xrray-polyfill.js' type='module'></script>
    <script src='js/webxr-polyfill.js'></script>

    <script src='js/webxr-button.js'></script>
  </head>
  <body>
    <header>
      <details open>
        <summary>Reduced Bind Rendering</summary>
        <p>
          This sample demonstrates a simple technique to reduce the number of
          state changes an application needs to make while rendering, potentially
          enabling better performance.
          <a class="back" href="./index.html">Back</a>
        </p>
      </details>
    </header>
    <script type="module">
      import {WebXRView, Scene} from './js/cottontail/src/scenes/scene.js';
      import {Renderer, createWebGLContext} from './js/cottontail/src/core/renderer.js';
      import {QueryArgs} from './js/cottontail/src/util/query-args.js';
      import {CubeSeaNode} from './js/cottontail/src/nodes/cube-sea.js';
      import {FallbackHelper} from './js/cottontail/src/util/fallback-helper.js';

      // If requested, initialize the WebXR polyfill
      if (QueryArgs.getBool('allowPolyfill', false)) {
        var polyfill = new WebXRPolyfill();
      }

      // XR globals.
      let xrButton = null;
      let xrImmersiveRefSpace = null;
      let xrNonImmersiveRefSpace = null;

      // WebGL scene globals.
      let gl = null;
      let renderer = null;
      let scene = new Scene();
      scene.addNode(new CubeSeaNode());

      function initXR() {
        xrButton = new XRDeviceButton({
          onRequestSession: onRequestSession,
          onEndSession: onEndSession,
          supportedSessionTypes: ['immersive-vr']
        });
        document.querySelector('header').appendChild(xrButton.domElement);

        if (navigator.xr) {
          requestInlineSession();
        } else {
          initFallback();
        }
      }

      // This kicks off a really simple fallback rendering loop so that we don't
      // have to clutter up the sample code with fallback rendering logic. This
      // variant also has the benefit of including simple support for looking
      // around via clicking and dragging with the right mouse button or a two-
      // finger drag.
      function initFallback() {
        initGL();
        let fallbackHelper = new FallbackHelper(scene, gl);
      }

      function initGL() {
        if (gl)
          return;

        gl = createWebGLContext({
          xrCompatible: true
        });
        document.body.appendChild(gl.canvas);

        function onResize () {
          gl.canvas.width = (gl.canvas.offsetWidth * window.devicePixelRatio);
          gl.canvas.height = (gl.canvas.offsetHeight * window.devicePixelRatio);
        }
        window.addEventListener('resize', onResize);
        onResize();

        // Set up a non-black clear color so that we can see if something renders wrong.
        gl.clearColor(0.1, 0.2, 0.3, 1.0);

        renderer = new Renderer(gl);

        scene.setRenderer(renderer);
      }

      function onRequestSession() {
        navigator.xr.requestSession('immersive-vr', {requiredFeatures: ['local']}).then((session) => {
          session.mode = 'immersive-vr';
          xrButton.setSession(session);
          onSessionStarted(session);
        });
      }

      function requestInlineSession(options) {
        return navigator.xr.requestSession('inline', options)
          .then((session) => {
            session.mode = 'inline';
            return onSessionStarted(session);
          });
      }

      function onSessionStarted(session) {
        session.addEventListener('end', onSessionEnded);

        initGL();

        session.updateRenderState({
            baseLayer: new XRWebGLLayer(session, gl)
        });

        session.requestReferenceSpace('local').then((refSpace) => {
          return refSpace;
        }, (e) => {
          if (!session.mode.startsWith('immersive')) {
            // If we're in inline mode, our underlying platform may not support
            // the stationary reference space, but an identity space is guaranteed.
            return session.requestReferenceSpace('viewer');
          } else {
            throw e;
          }
        }).then((refSpace) => {
          if (session.mode.startsWith('immersive')) {
            xrImmersiveRefSpace = refSpace;
          } else {
            xrNonImmersiveRefSpace = refSpace;
          }
          session.requestAnimationFrame(onXRFrame);
        });

        return session;
      }

      function onEndSession(session) {
        session.end();
      }

      function onSessionEnded(event) {
        if (event.session.mode.startsWith('immersive')) {
          xrButton.setSession(null);
        }
      }

      function onXRFrame(t, frame) {
        let session = frame.session;
        let refSpace = session.mode.startsWith('immersive') ?
                         xrImmersiveRefSpace :
                         xrNonImmersiveRefSpace;
        let pose = frame.getViewerPose(refSpace);

        scene.startFrame();

        session.requestAnimationFrame(onXRFrame);

        if (pose) {
          gl.bindFramebuffer(gl.FRAMEBUFFER, session.renderState.baseLayer.framebuffer);
          gl.clear(gl.COLOR_BUFFER_BIT | gl.DEPTH_BUFFER_BIT);

          // This is a different rendering pattern than the previous samples
          // used, but it should be more efficent. It's very common for apps
          // being ported to XR to take existing 2D rendering code and call the
          // top-level "drawScene" function once per XR view, effectively
          // drawing a single eye at a time. However, this causes many state
          // changes to be duplicated, which adds to the render loop's overhead.
          // By providing the matrices and viewports as an array to the drawing
          // function it can do all the necessary binding once and then call the
          // actual draw commands in a tighter loop, only changing the matrices
          // and viewport each time. This does mean that the viewport is changed
          // much more frequently (N times per object instead of N times per
          // scene) but it's typically a pretty cheap thing to change and will
          // almost always be easily outweighed by the savings from not
          // redundantly binding everything else.

          // For example, a traditional draw loop would do this:

          // Draw(views):
          //   for each view in views:
          //     setViewport();
          //     for each object in scene:
          //       bindProgram();
          //       bindMatrices();
          //       bindUniforms();
          //       bindBuffers();
          //       bindTextures();
          //       draw();

          // While this method results in a loop more like this:

          // Draw(views):
          //   for each object in scene:
          //     bindProgram();
          //     bindUniforms();
          //     bindBuffers();
          //     bindTextures();
          //     for each view in views:
          //       setViewport();
          //       bindMatrices();
          //       draw();

          // Note that for the complexity of the scene in this samples this
          // won't make much visible performance difference, but we're using the
          // more efficient pattern anyway as a way of promoting best practices.

          let views = [];
          for (let view of pose.views) {
            // Gather all the values needed for one view and push it into the
            // array of views to be drawn. WebXRView is a utility class that
            // holds all the necessary values for drawing a single view.
            let renderView = new WebXRView();

            // In future samples we'll hide this part away as well by using the
            // scene.drawXRViews() function, which handles gathering these
            // values internally.
            renderView.projectionMatrix = view.projectionMatrix;
            renderView.viewMatrix = view.transform.inverse.matrix;
            renderView.viewport = session.renderState.baseLayer.getViewport(view);
            views.push(renderView);
          }

          scene.drawViewArray(views);
        }

        scene.endFrame();
      }

      initXR();
    </script>
  </body>
</html>
